Biological waste made safe

With many agencies now engaged in the handling of biological agents, there has been significant investment in new facilities and in the modernising and upgrading of existing installations. Amongst the drivers for this investment has been: the boom in biopharmaceutical research, development and manufacture; recent health scares such as SARS and Avian Flu; and fears of bio-terrorism.

In many cases, the biological agents handled in these facilities have the potential to cause harm to people and to the environment and so effective management of risk is of paramount importance. As well as eliminating potential exposure to operating staff, effective controls must ensure no loss of containment of biological agents to the environment.

Facility design and operating regimes are essential components in managing the risk of contamination of both employees and of the environment. Controls need to address:

• Personnel
• Premises
• Equipment
• Standard Operating Procedures
• Waste Disposal
• Storage of pathogens
• Physical security of premises

Regulatory framework

There is a relatively complex regulatory framework governing the handling of biological agents. The primary legislation governing operating companies and agencies is the Health and Safety at Work Act 1974 and the Animal Health Act 1981. Secondary regulations and codes of practice include:

• Control of Substances Hazardous to Health Regulations 2000
• Management of Health and Safety at Work Regu-lations 1999
• Genetically Modified Organisms (contained use) Regulations 2000
• Specified Animal Pathogens Order 1998

The key determinant of the facility design, construction and operation is the hazard classification of the biological agents to be handled. The classification is based on an organism’s ability to cause disease by infection: Hazard Group 1 being the least dangerous and Hazard Group 4 being the most dangerous. The hazard classification of the biological agent is used to determine the containment level to be implemented throughout the laboratory facility – including the liquid effluent treatment system.

Principles of operation

Although effluent kill systems can be designed to operate on a continuous basis, most operate batch wise. There are two types of kill system in common use: chemical deactivation and heat sterilisation. Occasionally, a combination of these is deployed. The heat-sterilised systems can be split into two types, direct steam injection and indirect heating.

The complexity of the kill system and processing techniques required to treat a particular waste stream depends upon a number of features as described below. Systems can be as basic as a simple plastic tank to which bleach is added and the contents re-circulated prior to discharge, or they can be extremely complex with multiple steps and automated to cover all eventualities.

The batch systems operate in a similar fashion. Effluent is collected and batched into a deactivation tank where the pathogens are killed by either chemical breakdown or heat. A simplified flowsheet is shown below for a batch system using indirect steam to the deactivation vessel jacket.

Design considerations

The principle design consideration is the classification and nature of the organism to be treated. Each of the biological agent hazard groups requires a different level of containment and each organism may require different methods of treatment for effective deactivation.

Thereafter, the treatment plant design must be consistent with the laboratory operating regime. Other considerations include:

• Total effluent volume
• Maximum instantaneous flow rate
• Operating times: continuous or day shift operation
• Presence of solids such as human and animal wastes
• Chemicals used in the laboratory
• Materials of construction
• Level of automation required

The mechanical design of the treatment system should aim to minimise the risk of fugitive emissions from the plant. Wherever possible, systems tend to have the dangerous materials fed under gravity to collection tanks in a protected and controlled area. If practicable the killing of the pathogens will be done in the collection tank to minimise the transport of the material and hence reduce the risk of leakage in pumped systems. Where pumps are unavailable then protected sumps are used to ensure that material cannot escape.

To remove the risk of emissions to atmosphere the tanks are vented through a scrubbing system or by using high integrity in line filters or both. In turn the filters must be capable of being sterilised to allow for maintenance. For systems with separate collection and deactivation tanks then the number of filter systems can be reduced by use of common vent headers.

The utility requirements for the treatment system must be calculated and catered for. When a system is modified, for example, as a result of the addition of additional pathogens to an existing facility the existing utilities may influence the system choice.

Direct steam injection into the waste stream within a deactivation tank allows more rapid processing than a similar system using indirect steam heating on a jacket. However the total quantity of effluent leaving the site will be increased and this may be important for those facilities where waste disposal costs are volume related.

Whilst a slower processing method, the use of indirect steam heating via a vessel jacket is more energy efficient as the condensate can be collected and fed back to the steam boiler for reuse.

Both of the heating techniques described above require a post treatment cooling system to reduce the deactivated waste temperature prior to discharge to meet the conditions of many UK water companies.

A further consideration, particularly for containment level 3 and 4 facilities where the treatment plants tend to be housed in contained areas, is the impact of hot tanks on the building HVAC systems.

Chemical deactivation systems have the benefit of not requiring heating or cooling to be provided. However, wastes with solid matter may not be treatable. Additionally the use of chemical treatment provides additional design and operating considerations such as storage and handling of chemicals and generally requires a greater level of manual intervention to operate.

Finally, whatever technique is chosen, the system must be designed with maintenance in mind. It must be possible to shut down the system safely and to perform the maintenance activity and subsequent re-commissioning safely. Every surface where pathogens may contact has to be treated so that there is no risk to maintenance staff. The common means of achieving this is using steam sterilisation. This requirement is a factor that tends to drive the system selection towards using heat through steam.

Once the system has been designed and installed, performance must be validated. This is to ensure that it performs as intended, that is, that the biological agents for which the system was designed are actually destroyed by the treatment plant.

Building on experience gained on projects for research institutions, biotechnology and bio-manufacturing companies, Excelsyn recently designed, built and commissioned a SAPO (Specified Animal Pathogens Order) containment level 4 effluent kill system for a laboratory handling Avian Flu virus. Excelsyn was engaged by Shepherd Construction which was contracted to provide a laboratory complex for the Veterinary Laboratories Agency in Surrey.

The facility was required to be designed, fabricated, installed and commissioned as a fast-track project within four months. The plant comprises an underground collection tank taking wastes from the laboratory complex by gravity. Waste material is then pumped into one of two tanks for treatment. Deactivation of the virus is by holding at 96ºC for a minimum of two hours with the heat source being indirect steam heating of the tank jacket. Following deactivation the material is cooled prior to discharge.

Excelsyn’s multi-disciplined engineering design and construction team worked closely with the main contractor and end-user client to ensure that the planned operating regime was built into the design. For example, the plant is automated to minimise operator requirements and to maximise repeatability of operation.

Excelsyn was able to create pre-assembled skid mounted units as key components of the system. To reduce the project lead-time, the skids were fabricated at Excelsyn’s dedicated workshop while the main contractor was completing building construction. In addition to a reduction in lead time, there was less overlap and disruption at the client’s site.

Key to the success of any project installing a biological effluent kill system is close collaboration between the end-user, the main contractor responsible for laboratory construction and the specialist effluent treatment company. Only where the regulatory framework is understood and the operating regime clear can the effluent system work seamlessly with the rest of the facility and not be the ‘tail wagging the dog’